Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers are typically formed of piezoelectric elements. The elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer assembly. The transducer assembly is then further assembled into a housing possibly including control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the PZT transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
One limitation of PZT devices, in ultrasonic imaging applications, is that the acoustic impedance is approximately 30-35 MRayls (kg/m.sup.2 s), while the acoustic impedance of the human body is approximately 1.5 MRayls. Because of this large impedance mismatch, acoustic matching layers are needed to match the PZT impedance to the body impedance. Acoustic matching layers work using a 1/4 wave resonance principle and are therefore narrow band devices, their presence thus reducing the available bandwidth of the PZT transducer. In order to achieve maximum resolution, it is desirable to operate at the highest possible frequency and the highest possible bandwidth.
In order to address the shortcomings of transducers made from piezo-electric materials, a micro-machined ultrasonic transducer (MUT), as described in U.S. Pat. No. 5,619,476 to Haller, et al., has been developed. Micro-machined ultrasonic transducers of this type address the shortcomings of PZT transducers by, among other attributes, being fabricated using semi-conductor fabrication techniques on a silicon substrate. The MUT's are formed using known semiconductor manufacturing techniques resulting in a capacitive non-linear ultrasonic transducer that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. By applying contact material to the membrane, or a portion of the membrane, and to the silicon substrate and then by applying appropriate voltage signals to the contacts, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the membrane, which then generates a receive signal. When imaging the human body, the membrane of the MUT moves freely with the imaging medium, thus eliminating the need for acoustic matching layers. Therefore, transducer bandwidth is greatly improved.
PZT transducers have a generally linear relationship between acoustic pressure and applied voltage. As such, this linear relationship preserves the harmonic nature of the applied voltage waveform.
When using a MUT in an ultrasonic imaging application, such as harmonic imaging, it is desirable to excite the MUT to create an input pulse using an input frequency (f1) and then use the MUT to receive reflected energy at a receive frequency (f2). Typically, the input frequency is in the range of 1.8 MHz and the receive frequency is in the range of 3.6 MHz. A human body, as well as ultrasonic contrast agents, which may be injected to enhance an ultrasonic image, are non-linear so that when imaging human tissue, the body reflects an ultrasonic input pulse at a frequency twice that of the input pulse. One of the shortcomings of MUT's, however, is that they have a non-linear relationship between voltage and pressure, as shown by the following equation: EQU P=eV.sup.2 /2(x+T.backslash.Er).sup.2, (Eq.1)
where V=applied voltage, PA1 X=MUT vacuum gap thickness, PA1 e=permittivity constant, PA1 T=thickness of MUT membrane (top and bottom combined), PA1 Er=relative dielectric constant of MUT membrane.
The non-linear relationship between acoustic pressure and applied voltage in a MUT produces a distorted pressure waveform resulting in a spectrum having unacceptably high second order harmonic energy. This second harmonic energy on the transmit pulse interferes with the image reflected by the tissue under analysis resulting in both f2 being received at the MUT and the reflection of f2 (that of the second harmonic energy appearing as a linear component) being received. This condition results in both a linear component being received and a non-linear component being received.
Therefore it would be desirable for a MUT to produce a non-distorted pressure waveform having maximum possible second order harmonic rejection characteristics.